Evaluation and application of the ion microprobe in the strontium isotope geochemistry of carbonates

A modified AEI-IM20 ion microprobe has been used to measure 87Sr/86Sr ratios in carbonates. A suite of carbonates with varying major elements (Ca, Mg, Fe, Mn) was studied at low and high (M/ΔM approx.= 3000) mass resolution to determine the types and intensities of molecular species isobaric with Sr peaks; Sr data collected at low mass resolution must be corrected for Ca2 and CaMgO species. Rb/Sr ratios are extremely low, and correction for 87Rb is not required (< 0.1per mille of 87Sr). Usable Sr isotopic data may be obtained from calcite given Sr >= 400 ppm, and for Sr > 5000 ppm a precision of approx.= +- 1per mille (+- 0.0007) in 87Sr/86Sr (2 sigma mean) can be achieved under optimum conditions. The corrections for Ca2 and CaMgO are smaller than the within-run precision in calcite, but in dolomite the correction for CaMgO is +1.5%. Mass fractionation corrections to 87Sr/86Sr (based on 86Sr/88Sr=0.1194) are typically +8 to +10per mille. Good agreement between ion probe and solid source mass spectrometer results was found for calcites of known Sr isotopic composition: St4a (approx.= 400 ppm Sr), average ion probe 87Sr/86Sr=0.7267 +- 0.0015, solid source 87Sr/86Sr=0.72680 +- 0.00008 [14]; JCG36 (approx.= 6000 ppm Sr), average ion probe 87Sr/86Sr=0.7056 +- 0.0009, solid source 87Sr/86Sr=0.70588 +- 0.00009 [16]; JCG44 (approx.= 6000 ppm Sr, average ion probe 87Sr/86Sr=0.7057 +- 0.0006, solid source 87Sr/86Sr=0.70540 +- 0.00008 [16]. The ability of the ion microprobe to measure 87Sr/86Sr for 10-μm spots in calcite was used: (1) no measure variation in 87Sr/86Sr of approx.= 0.01 on a centimetre scale in a hydrothermally altered basalt from the Isle of Skye, northwestern Scotland; and (2) to determine the Sr isotopic composition of tiny (< 35 μm) calcite grains in a veined mantle Iherzolite from Bultfontein, South Africa. Because of calcite's ubiquitous occurrence in many parageneses this technique may offer many opportunities for the measurement of fine scale heterogeneities in 87Sr/86Sr. (orig.)